Before integrated circuits (ICs), sometimes termed chips or dies in the art, are attached to a printed circuit board, they are packaged by encapsulation in a polymer material. The encapsulation process is designed to protect the individual IC during handling and operation. Typically, the process used involves a suitable polymer that is injected in a liquid-phase into a mold cavity that contains the IC to be encapsulated. During the injection process, the mold is held closed by substantial pressure. As this process is part of a manufacturing operation, many ICs of a common type are typically processed at the same time, using the same molding fixture. After the encapsulation process is completed and the polymer has solidified, the mold is separated. Finished parts are then collected for further processing that may be needed before they can be mounted to a printed circuit board (PCB).
There are many different types of integrated circuits. There are variations in shape, size, and configuration. Therefore, there are differences in techniques for encapsulation. Some ICs are encapsulated using a process called Lead Frame Injection designed to facilitate Surface Mount Technology (SMT) which is a method for attaching an IC package to a PCB. There are some types of packages wherein an IC is mounted to one side of a substrate, and encapsulation is performed to protect the IC on the side of the substrate to which the IC is mounted. These and other molding processes are well known in the art and are typically automated processes geared for mass production.
For the purpose of this specification, injection molding of the type used in IC encapsulation for one-sided-molded packages, such as for BGA packages, will be solely addressed hereafter, as this is the type of process that is relevant to the present invention.
In the process of making a BGA and some other types of packages incorporating one-side molding, an IC is typically attached to the top surface of a substrate material by means of a suitable adhesive that can withstand process temperatures. The ICs are typically rectangular in shape with an area footprint that is somewhat smaller than the area of the substrate to which they are mounted. While it is not specifically required, the respective geometric shapes of the IC and the substrate are generally the same.
Substrates used in forming one-side-molded IC packages typically vary in thickness depending, in part, upon the specific application for which the ICs are designed. In a typical one-side molding operation, the thickness of a substrate plays a critical role in the success of the process. This is because, in a typical operation, one of the mold plates has to match the substrate thickness, as will be apparent in further description below.
Molds of the type used in one-side molding encapsulation typically comprise four separate plates that are adapted to be held in alignment with respect to one another. Generally, there is a base plate, a package locator plate, a cavity plate, and a top plate. It is the package locator plate that is typically sized in thickness to the thickness of the substrate upon which an IC is mounted. These mold plates may be fabricated from heat-treated tool steel, enabling the mold assembly to withstand substantial temperature and pressure, and to resist wear and damage.
In conventional applications of IC encapsulation, the substrate rests flat on a base plate. A package locator plate has a matrix of openings adapted to receive and position substrates. The locator plate is of a thickness to allow a cavity plate with smaller openings to sit flush against the substrates and the top surface of the locator plate at the same time, making contact with both surfaces in the same plane. The IC, which is mounted to the substrate, is thus positioned within an opening in the cavity plate. A top plate sits flush on the cavity plate, which is thicker than the substrate, completing a cavity enclosure. Liquid phase polymer injected into the cavities thus formed encapsulates the ICs mounted to the substrates.
As described above, thickness of the substrate to which the IC is mounted to is critical. In a mold set provided for molding packages wherein substrate thickness varies, several alternate locator plates must be maintained to be substituted in the mold set to accommodate change in substrate thickness.
What is clearly needed is a method and apparatus wherein one locator plate of a single thickness may be used in one-side package molding wherein the substrate thickness may vary, thereby eliminating the need for batch-specific locator plates, time consuming inspection processes, and also reducing periodic mold plate modification requirements.